Oroxylin A, which is mainly found in Scutellaria baicalensis GEORGI and Oroxylum indicum and characterized by specific functional groups arrangement at C5, C6 and C7, is a flavonoid having the following formula (I), in which the functional group at either one of C5 and C7 is a hydroxyl group, and that at C6 is a methoxyl group. Much attention has been paid to the pharmacological importance of oroxylin A due to its wide physiological activities (such as pressure reduction, anti-tumor action, neuro-protection, anti-inflammation, etc.).

In general, oroxylin A can be extracted from plants, e.g., Scutellaria baicalensis, Oroxylum indicum, Capparis spinosa, Ardisia crispa, Eucommia ulmoie OLIV, etc. However, the extraction process applied is time-consuming, cost-ineffective and unproductive. Therefore, various methods for synthesizing oroxylin A have been developed and published in articles.
In a previous study, W. H. Huang et al. reported a method for the synthesis of flavonoids (including oroxylin A) of Scutellaria baicalensis GEORGI, which employs trimethoxyphenol as the starting material (Huang, W. H. et al. (2003), Chem. Pharm. Bull., 51:339-340). However, the result regarding the synthesis of oroxylin A was proven to be wrong in a later study (Panhekar, D. et al. (2015), Pharm. Res., 7:174-180). In fact, the actual compound synthesized in W. H. Huang et al. (2003), supra should be negletein of the following formula (II) rather than oroxylin A. This finding indicates that it is quite a challenge to correctly prepare oroxylin A using a chemical synthesis method.

In addition, Kostrzewa-Susłow et al. reported microbial transformation of baicalein (5,6,7-trihydroxyflavone) and baicalin (baicalein-7-O-glucuronide) into oroxylin A after six days of incubation with Chaetomium sp. (Kostrzewa-Susłowa, E. et al. (2007), J. of Mol. Catal. B: Enzym., 49:113-117).
Chen disclosed use of 2,4,6-trinitrotoluene as a starting material for synthesizing oroxylin A (Chen, C. P. (2008), Yearbook of Chinese Medicine and Pharmacy, 26:241-358).
CN 101508689 B disclosed a method for synthesizing oroxylin A using baicalein as the starting material. The method includes the steps of: (a) adding a protecting group to C7 position of baicalein; (b) methylating the hydroxyl group at C6 position; and (c) removing the protecting group at C7 position to obtain oroxylin A.
It is noted that the aforesaid chemical synthesis methods are complicated and the reaction conditions required are stringent, and the above-mentioned microbial transformation is time-consuming.